Wireless radio links have become increasingly important to provide data communication links for a variety of applications. For example, Internet Service Providers have begun to utilize wireless radio links within urban settings to avoid the installation expense of traditional wired connections or optical fiber. The typical architecture of such wireless systems involves establishing an outdoor unit at a node of the wireless system. The outdoor unit contains an antenna and a transceiver. Additionally, the radio unit is connected via a cable to an indoor unit. The indoor unit contains a modem to recover the digital data contained within the received signal and to add digital data to a carrier for ultimate transmission by the radio unit. The separation of functionality between indoor units and outdoor units has been maintained for several reasons. Among these reasons, maintenance of an indoor unit facilitates greater thermal stability of the modem unit and further increases reliability of the communication link. Additionally, the communication channel between the indoor unit and the outdoor unit is often allocated utilizing a Time Division Duplex (TDD) scheme, i.e. the communication over the cable successively alternates between receive and transmit modes.
By maintaining separate modem and radio functionality and utilizing a TDD scheme, these systems require some method of synchronizing the modem and radio units. The synchronization mechanism ensures that both units will be in receive and transmit modes at the proper times. Known systems utilize a unique word (a series of pre-defined digital symbols for detection by the radio unit) to derive a frame pulse. Each frame pulse results in a counter being reset. The radio unit is preprogrammed to define transmit and receive time slots based upon this counter. Accordingly, the modem is capable of causing the radio unit to synchronize to the modem by signaling the unique word. Additionally, dynamic variations in transmit and receive slots may be implemented by signaling time slot parameters via a control channel.
The use of a unique word and time slot parameters suffers several limitations. First, the use of a unique word and time slot parameters prevents rapid variation in time slot duration. Specifically, the bandwidth utilized to signal the unique word as well as the time slot parameters is not sufficient to permit dynamic allocation of the communication channel on a millisecond by millisecond basis. It is anticipated that more efficient channel access methods will be required for point-to-multipoint systems as these systems provide services to a greater number of users. Accordingly, dynamic and rapid variation of time slots is a valuable mechanism for maximizing the efficiency of such access methods. Additionally, the use of a unique word is problematic, since the timing of the system relies upon the frame pulse. If a unique word is not detected and a frame pulse is consequently missed, the timing information may become skewed which could cause the communication link to be broken. Accordingly, unique word synchronization involves a degree of inherent unreliability.
The present invention is directed to a system and method which selectively enable and disable the transmit and receive functions of a radio. In the preferred embodiment, a modem and a radio are remotely located. The modem and radio preferably communicate via an IF signal over a communication channel in a Time Division Duplex manner, i.e. the same communication channel is alternatively allocated as a transmit path and a receive path. Additionally, the modem preferably creates an unique frequency as a discrete out-of-band tone. The transmit path of the radio preferably filters the tone from the received signal. The radio preferably contains a separate mechanism to detect the out-of-band tone. This mechanism converts the out-of-band tone to a logical signal. If the logical signal is present, the radio operates through the transmit path. Preferably, the radio defaults to the receive path if the logical signal is not present.
According, certain embodiments of the present invention provide a more robust mechanism for dynamically controlling transmit and receive functions between separate modem and radio modules. Certain embodiments the present invention provide a more easily detected signaling mechanism for dynamically controlling the transmit and receive functions between separate modem and radio modules. Also, certain embodiments of the invention provide a mechanism for dynamically controlling transmit and receive functions between separate modem and radio modules that requires minimal switching times.
One embodiment of the present invention avoids the problems of the prior art by providing a control signal from the modem to the transceiver where said control signal is substantially coextensive with the communication signal being sent by the modem to the transceiver for transmission. The control signal and the communication signal may be sent over the same cable connecting the modem and the transceiver. In the situation where the control signal and the communication signal are sent over the same cable, the control signal preferably is at a frequency different than the frequency of the communication signal so as to avoid material interference between the two signals. The control signal may be a tone.
In a preferred embodiment, the transceiver is normally in receive mode and is in transmit mode for the duration of the control signal. The control signal is not sent from the modem as a part of the communication signal to set the transmit and receive slots via a counter thereby taking up valuable bits in the communication signal and thereby delaying the switching of the transceiver between modes. The control signal, unlike a unique word in the prior art, does not set a counter which defines the transmit slot of the transceiver.
Accordingly, it is an object of the present invention to provide a more robust mechanism for dynamically controlling transmit and receive functions between separate modem and radio modules.
It is another object of the present invention to provide a more easily detected signaling mechanism dynamically controlling the transmit and receive functions between separate modem and radio modules.
It is yet another object of the present invention to provide a mechanism for dynamically controlling transmit and receive functions between separate modem and radio modules that require switching times.
It is still another object of the present invention to provide a control signal from a modem to a transceiver where the control signal is substantially coextensive with, and at a different frequency than, a communication signal to be transmitted.
It is a further object of the present invention to provide a control signal from a modem to a transceiver wherein the transceiver remains in the transmit mode only for the duration of the control signal.
It is yet a further object of the present invention to provide a control signal and a communication signal from a modem to a transceiver over the same cable at different frequencies.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.